The present invention relates to a method of arranging cyclic patterns in a tire tread, more particularly to a method of determining a phase shift between variable-pitch sequences in axially different circumferential regions.
In general, a tread pattern of a tire defined by tread grooves provided in the tread portion of the tire is formed by repeating a design cycle in the circumferential direction of the tire while changing the circumferential pitch length of the design cycle as well known in the art as variable pitch methods. In order to avoid simultaneity in ground contact, such a method that one half of the tread pattern on one side of the tire equator is circumferentially shifted from the other half by about one half of a pitch length is widely employed.
Although this method is effective in reducing a tire noise, it is not always effective in improving other performance of the tire such as vibration during high speed running, tire uniformity and the like. For example, there is a possibility that the tangential force variation TFV, radial force variation RFV and the like become rather worse in view of vibration during high speed running. When the periodicity of such a force variation is synchronized with the natural vibration of the suspensions, spring members or the like of the car at a certain running speed, even if the amplitude thereof is small, a vibration which can be felt is caused. Such a phenomenon is liable to occur at higher harmonics such as 2nd and 4th harmonics of the tangential force variation.
It is therefore, an object of the present invention to provide a method of arranging cyclic patterns in a tire tread which can improve the tire uniformity causing vibration as well as noise performance.
According to the present invention, in a method of arranging cyclic patterns in a tire tread, wherein the tire tread comprises a number (P) of circumferential regions each provided with a cyclic pattern formed by circumferentially repeating a number (I) of design cycles having a number (J) of different pitch lengths, and the pitch lengths of the design cycles in each said circumferential region are arranged circumferentially of the tire in a sequence whereby the number (P) of sequences of the pitch lengths are defined, the improvement comprises:
obtaining, with respect to each of the design cycles, an averaged pitch of the pitch lengths of a number (n) of circumferentially subsequent design cycles to define a sequence of averaged pitches per each of the circumferential regions, whereby the number (P) of sequences of the averaged pitches are defined;
defining a number (Q) of combinations of the number (P) of the sequences of averaged pitches by changing phase between the sequences;
obtaining, with respect to the averaged pitches of the number (P) of the combined sequences in each of the combinations, a crossover average of averaged pitches, whereby a series of the crossover averages is defined per each of the combination;
obtaining, from said a series of the crossover averages, a difference between a maximum and a minimum of the crossover averages or alternatively between a maximum and a minimum of the amplitude of a harmonic of said a series of the crossover averages, with respect to each of the combinations;
finding, from the differences of the combinations, a combination having a smaller difference to determine the phase shift given to the sequences of averaged pitches in making the combination; and
rearranging the cyclic patterns of the circumferential regions in accordance with the found phase shift.